Water-absorbing resins are widely used in sanitary and hygienic goods, wiping cloths, water-retaining agents, dehydrating agents, sludge coagulants, disposable towels and bath mats, disposable door mats, thickening agents, disposable litter mats for pets, condensation-preventing agents, and release control agents for various chemicals. Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles.
Such water-absorbing resins are termed “superabsorbent polymers,” or SAPs, and typically are lightly crosslinked hydrophilic polymers. SAPs are discussed generally in U.S. Pat. Nos. 5,669,894 and 5,559,335, each incorporated herein by reference. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirement for an SAP used in a hygienic article, such as a diaper.
As used herein, the term “SAP particles” refers to superabsorbent polymer particles in the dry state, more specifically, particles containing from no water up to about 10%, by weight, water. The terms “SAP gel,” “SAP hydrogel,” or “hydrogel” refer to a superabsorbent polymer containing at least about 10%, by weight, water, and often, particles that have absorbed at least their weight in water, and typically several times their weight in water.
SAPs have a tendency to degrade in color after long periods of storage. The tendency of an SAP to undergo a color transition from a clean, crisp, white color to a honey brown color accelerates as storage time, temperature, and humidity increase. In temperate climates, such as the United States and Europe, the rate at which an SAP undergoes color degradation is sufficiently slow such that the SAP, or article containing the SAP, typically is consumed before a color change is observable to the naked eye.
However, in tropical and subtropical climates, such as in South America and Southeast Asia, SAP color degradation is sufficiently rapid such that a color change often occurs before the SAP, or article containing the SAP, is consumed. In areas like Southeast Asia, an SAP can change color from white to honey brown in about 4 to 6 weeks. This problem is exacerbated because the SAPs may be produced far from the tropical climate, thereby increasing the time span from SAP production to use. Furthermore, consumption of articles containing an SAP in such climates is relatively low, therefore further increasing the time period between SAP production and use.
The change in color of the SAP does not affect SAP performance, but adversely affects consumer acceptance of articles containing the color-degraded SAPs. In particular, consumers observing a color-degraded SAP in a diaper form an opinion that the diaper contains a contaminant, is somehow soiled or faulty, or is of low quality. The diaper typically is returned for a refund, and the consumer is less likely to repurchase that brand of diaper.
Problems also arise at the manufacturing level because manufacturers of diapers and other articles containing an SAP refuse to incorporate a discolored SAP into their products, and return the discolored SAP to the SAP manufacturer. A color-degraded SAP, therefore, ultimately adversely affects the manufacturer of articles and the manufacturer of the SAP, who must absorb the cost of the returned goods.
It would be desirable to provide an SAP that exhibits exceptional color stability properties, such that the SAP retains its crisp, white color throughout the useful life of the SAP, or an article containing the SAP, even when stored under high temperature and humidity conditions. Furthermore, it would be desirable to provide an SAP having a long-term color stability and low residual monomer content, without adversely affecting the absorbent properties of the SAP, such as absorbing a large amount of liquids quickly, having a good fluid permeability into and through the SAP, and having a high gel strength, such that an SAP hydrogel formed from the SAP does not deform or flow under an applied stress or pressure.
Currently, SAPs, like partially neutralized, lightly crosslinked, polyacrylic acid, are manufactured using a persulfate as a component of the polymerization initiator system. A persulfate is included in the initiator system as the oxidizing agent of a redox initiator pair and to reduce the amount of residual acrylic acid monomer in the SAP to acceptable levels. A persulfate also can act as a thermal initiator. However, a persulfate interacts with the MEHQ inhibitor present in acrylic acid monomer and imparts a low initial color to the SAP. This low initial SAP color progresses to a severe SAP discoloration over time, and especially under high temperature and humidity conditions.
The present invention is directed to overcoming the problem of SAP discoloration attributed to the presence of a persulfate in the preparation of an SAP. As discussed in detail hereafter, the present invention overcomes the SAP discoloration problem by essentially omitting a persulfate from the polymerization initiator system of a monomer mixture, and by subjecting the SAP hydrogel resulting from the polymerization to a low dose of ultraviolet (UV) radiation.
Ultraviolet radiation previously has been used in the preparation of SAPs. For example, UV radiation has been used in conjunction with a photoinitiator to initiate polymerization of monomers to form an SAP hydrogel, as disclosed in EP 0 290 814 B1. DE 41 23 889 A1 discloses UV irradiation of a water-absorbing resin prepared from a water-soluble polymer and a polysaccharide and/or crosslinking agent, in the presence of a radical scavenger, to provide a water-absorbing resin having a low amount of water-soluble components (7 wt % or less) and a low amount of residual monomer (500 ppm or less). The UV radiation is applied during drying or crushing of the water-absorbing resin.
PCT publication WO 01/55228 discloses subjecting a water-soluble or water-swellable polymer to UV radiation to reduce residual monomer content. An ultraviolet initiator is used in an amount of up to 10,000 ppm, by weight of monomers, preferably up to 5000 ppm, more preferably 50 to 3,000 ppm, and still more preferably 500 to 2,000 ppm. UV radiation typically is conducted for about 20 minutes.
PCT publication WO 01/25289 discloses subjecting an acrylic polymer to UV radiation after, or simultaneously with, comminuting a gelled polymer to gelled polymer particles. In particular, the comminuted gel particles can be irradiated during a drying step in a fluid bed dryer.